The present invention relates to a process for the preparation of high purity alpha mercuric iodide for use as a starting material source for the growth of alpha mercuric iodide monocrystals usable for the production of X and gamma radiation spectrometers and detectors operating at ambient temperature.
For the production of X and gamma radiation spectrophotometers and detectors, it is necessary to have high purity alpha mercuric iodide in order to bring about the growth of undeformed .alpha.-HgI.sub.2 monocrystals, which have a high degree of purity and a perfectly controlled stoichiometry. To obtain satisfactory performances, it is important to ensure that the monocrystals do not have trapping centres with deep levels in the forbidden band, because the charge carriers can be trapped by deep level centres. The latter are generally due to an error with respect to the stoichiometry of the monocrystal, to plastic deformations (dislocations) and to the inclusion of impurities produced during monocrystal growth.
A process for the production of .alpha.-HgI.sub.2 monocrystals is known, which makes it possible to both control the stoichiometry of the monocrystal and to minimize plastic deformations. This process, described in French patent application No. 7916418 of June 26th, 1979, uses as the starting substance alpha mercuric iodide polycrystals and a solution of HgI.sub.2 in an organic sulphoxide and methanol, which has previously been purified by electrolysis.
Although this process leads to the obtaining of crystals with satisfactory properties, it is desirable to still further improve the properties of the monocrystals by using polycrystals having a high purity level for growth purposes, because in the polycrystals obtained by conventional synthesis processes, from elements having a high purity level, the content of impurities of a metallic, metalloid and organic nature is still too high.
A process for the purification of .alpha.-HgI.sub.2 is also known, which is intended to be used for the growth of high quality crystal. This process, described in French patent application No. 2,316,192 comprises a first stage of synthesizing the mercuric iodide from mercury and iodine, a second purification stage involving the repeated sublimation of the mercuric iodide prepared by synthesis and a third purification stage involving zone melting of the mercuric iodide purified by sublimation.
However, this process suffers from numerous disadvantages.
(a) It is impossible to obtain large quantities of .alpha.-HgI.sub.2 during dry synthesis, due to the risk of explosions and consequently the .alpha.-HgI.sub.2 quantities prepared by this process are limited to 100 g per charge.
(b) The large number of sublimation and zone melting stages gives rise to performance difficulties and gives a low efficiency level and yield. To obtain a high degree of purity, it is necessary to carry out 6 to 30 successive sublimations and 30 to 100 zone melting operations.
(c) The efficiency of impurity separation is low in the sublimation stage because, at the sublimation temperature between 100.degree. and 140.degree. C., most of the organic impurities form stable complexes with .alpha.-HgI.sub.2, which have a vapour pressure comparable to that of .alpha.-HgI.sub.2. Therefore, these complexes sublimate and condense at the same time as the .alpha.-HgI.sub.2. In the same way, the organic impurities only partly decompose at the temperature used for the sublimation. Thus, analyses by mass spectrometry have revealed that the alpha mercuric iodide monocrystals purified by repeated sublimations and obtained by vapour phase growth still contain at least 10 elements (C, N, O, Al, Ca, Mn, Na, K, Cr and Fe) and the organic impurities are broken down into 7 hydrocarbon radicals (CH.sub.3, C.sub.2 H.sub.3, C.sub.3 H.sub.3, C.sub.3 H.sub.4, C.sub.4 H.sub.4, C.sub.4 H.sub.7 and C.sub.4 H.sub.8) at relatively high concentrations, because on average they exceed 50 to 100 ppm atomic for each element or hydrocarbon radical.
(d) The zone melting is difficult to carry out, due to the high vapour pressure of melted HgI.sub.2, particularly as the HgI.sub.2 purified by sublimation contains a large amount of organic impurities which crack and give carbon and tar suspensions which cannot be separated by zone melting and gaseous products which, by overpressures, may shatter the sealed tube containing the HgI.sub.2 to be purified.
Another purification process is known consisting of a first stage of purification by recrystallization from a .alpha.-HgI.sub.2 -saturated HCl solution (1:1) at approximately 100.degree. C., a second stage of purification by "thermochemical" distillation (vacuum evaporation at approximately 200.degree. C., superheating the HgI.sub.2 vapour at 400.degree. to 500.degree. C. and condensation of the .alpha.-HgI.sub.2 at 40.degree. to 50.degree. C.) and a third purification stage by repeated sublimations at 120.degree. to 130.degree. C.
However, this process also has numerous disadvantages:
(a) the resulting product is polluted by chlorine,
(b) the resulting product is non-stoichiometric (Hg rich),
(c) the process requires 6 to 8 final sublimation cycles with a very low speed of 2 g/hour giving a total yield of 10 to 12%.